The present invention relates to a transgenic organism.
In particular, the present invention relates to a transgenic starch producing organism having an increased ability to synthesize starch and one that is capable of producing starch in high yields. More particularly the present invention relates to a transgenic organism comprising a nucleotide sequence coding for exogenous ADP glucose pyrophosphorylase (AGP).
In a preferred embodiment the present invention relates to a transgenic plant or plant cell capable of expressing exogenous AGP in the starch producing centres in the plant, namely the chloroplasts and the amyloplasts. The invention also relates to a recombinant DNA construct for use in the transformation of a plants or plant cell showing enhanced starch production, and plants and plant cells transformed with the recombinant DNA construct.
ADP glucose pyrophosphorylase (E.C.2.7.7.27) (AGP) is one of the primary enzymes involved in the biosynthesis of starch and glycogen in organisms such as plants, algae, fungi and bacteria, particularly plants.
AGP catalyses the reaction:
xcex1-glucose-1-P+ATP⇄ADP-glucose+PP1
the product ADP-glucose being the major donor of glucose in the biosynthesis of starch in plants. Moreover, that reaction has been shown to be the rate limiting factor in the synthesis of starch in organisms such as plants, the rate of that reaction in turn being critically dependent upon the AGP concentration. Because of that, AGP has been the subject of intensive investigation and for a general review of recent studies on plant AGP, reference should be made to Kleczkowski et al: Z. Naturforsch. 46c, 605-612 (1991).
As reported by Kleczkowski et al (ibid) and elsewhere, AGP is widely distributed throughout the plant kingdom and is found in some starch producing bacteria, such as E. coli. Plant AGP exists as a tetramer (210 to 240 kDa) composed of two small sub-units (50 to 55 kDa) and two large sub-units (51 to 60 kDa) in contrast to bacterial AGP which appears to consist of four units of equal size. AGP has also been shown to be produced in cyanobacteria and in algae, where its tetrameric structure is similar to that in plants, i.e. two large and two small sub-units, rather than the homotetrameric structure found in ordinary bacteria.
Because of the commercial importance of starch, primarily as a foodstuff but also as an important industrial chemical, AGP itself and recombinant DNA constructs containing DNA sequences encoding AGP for the transfection of plants and plant cells as a means of increasing plant AGP concentration and hence increased biosynthesis of starch in plants and increased starch yields, have formed the subject matter of several recently published patent applications.
For example, in EP-A-0368506 a method of extracting AGP from wheat leaf and wheat endosperm is disclosed. Also disclosed are the cDNA sequences encoding wheat leaf and wheat endosperm AGP, and various plasmids containing those sequences for subsequent insertion into plants to provide plants having an increased ability to synthesise starch, although that latter step is not described in detail, nor are any examples given of transgenic plants containing those constructs.
WO 91/19806 discloses transformed plant cells and plants having elevated levels of starch and starch biosynthesis achieved by incorporating into the plant genome a DNA construct comprising in sequence a plant promoter, a DNA sequence encoding a fusion polypeptide consisting of a plastid transit peptide and a bacterial (E. coli) AGP, and a 3xe2x80x2-non-translated region which functions in the plant cell to cause transcriptional termination and the addition of a polyadenylated tail to the 3xe2x80x2-end of the corresponding DNA sequence. The DNA sequence encoding E. coli AGP is given, as well as the deduced amino acid sequence. Transgenic potato and tomato plants transformed with the E. coli AGP gene are shown to produce increased starch yields. It is suggested that other bacterial sources besides E. coli, and also algae, may be used as a source for the AGP gene to be used in the transformation of the plants and plant cells to provide increased starch yields. However, there is no mention of the isolation of nucleotide sequences coding for AGP enzymes from those other sources or their expression in such transgenic systems.
A similar disclosure is contained in WO 92/11382 which likewise discloses the transformation of plants, especially potato plants, with bacterial (E. coli) DNA encoding bacterial AGP, with the objective of increasing starch biosynthesis and starch yield in such plants.
A slightly different objective is set out in EP-A-0455316. There the objective is to increase sugar and protein concentrations in plant-based foodstuffs at the expense of starch formation. That is achieved by incorporating into the plant genome a DNA sequence encoding AGP, but in an inverted orientation in the transformation vector. Transcription of the reversed sequence results in an anti-sense mRNA which inhibits the production of AGP in the plant cell leading to reduced AGP activity and reduced starch production.
All plant AGPs investigated so far have been reported to be strongly activated by 3-phospho-glycerate (PGA) and inhibited by inorganic phosphate (Pi). Also, the PGA/Pi ratio in the chloroplasts and amyloplasts where biosynthetic starch production is concentrated is believed to play a key regulatory role in starch synthesis. It is known, for example, that chloroplast PGA/Pi ratios are at the highest activity during the daylight hours, i.e. during photosynthesis, which period coincides with the peak period of starch production in the chloroplasts. The regulation of the AGP formation in non-photosynthetic tissues is less well understood, but the activatory and inhibitory roles of PGA and Pi, respectively, i.e. the PGA/Pi ratio, is believed still to play an important part.
The present invention addresses the problem of how to increase AGPase levels and/or starch levels in starch producing organisms.
According to a first aspect of the present invention there is provided a transgenic starch producing organism comprising a nucleotide sequence coding for an exogenous ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof which retains the enzymatic activity of the AGP enzyme, wherein the nucleotide sequence is capable of being expressed in the organism; characterised in that the activity of the enzyme or sub-unit thereof is substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of inorganic phosphate; and further characterised in that the activity of the enzyme or sub-unit thereof is not stimulated by fructose-1,6bisP and/or is not inhibited by AMP.
According to a second aspect of the present invention there is provided a transgenic starch producing organism comprising exogenous ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof which retains the enzymatic activity of the AGP enzyme, wherein the activity of the enzyme or sub-unit thereof is substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of inorganic phosphate and wherein the activity of the enzyme or sub-unit thereof is not stimulated by fructose-1,6-bisP and/or is not inhibited by AMP.
According to a third aspect of the present invention there is provided a potato tuber containing an enhanced starch content.
According to a fourth aspect of the present invention there is provided a method of increasing the rate and/or yield of starch production in an organism, especially a plant or a plant cell, which method comprises introducing into an organism a nucleotide sequence according to the present invention to form a transgenic organism according to the present invention and expressing the nucleotide sequence.
According to a fifth aspect of the present invention there is provided a method of increasing the rate and/or yield of starch production in an organism, especially a plant or a plant cell, which method comprises introducing into or forming in an organism a ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof according to the present invention.
According to a sixth aspect of the present invention there is provided any one of the following: A cDNA sequence identified herein as SEQ ID No. 2, including noncritical allelic variations of that sequence; An amino acid sequence as shown in SEQ ID No. 4, including variants thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence; A cDNA sequence identified herein as SEQ ID No. 5 including non-critical allelic variations of that sequence; An amino acid sequence as shown in SEQ ID No. 6, including variants thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
According to a seventh aspect of the present invention there is provided any one of the following plasmids: Plasmid pPPS1; Plasmid pPPL1; Plasmid pPPL1M; Plasmid pPPS4; Plasmid pPPL4; Plasmid pPPL5; Plasmid pBKL4; Plasmid pVictor IV GN.
According to an eighth aspect of the present invention there is provided a method of increasing the rate and/or yield of starch production in an organism, especially a plant or a plant cell, which method comprises introducing into an organism a recombinant DNA construct containing an exogenous DNA sequence encoding an exogenous ADP glucose pyrophosphorylase enzyme (AGP) or sub-unit thereof and one or more promoter sequences enabling the expression of the AGP encoded by that sequence by the organism thereby to increase the AGP content of the organism and in consequence to increase the rate of starch production by the organism and/or the starch yield, characterised in that the said DNA sequence is the gene sequence encoding the barley (Hordeum vulgare) endosperm AGP or a sub-unit thereof, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more points in the amino acid sequences defining the barley endosperm AGP or either of its sub-units, wherein the construct is expressed in the organism; characterised in that the activity of the enzyme or sub-unit thereof is substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of inorganic phosphate; and further characterised in that the activity of the enzyme or sub-unit thereof is not stimulated by fructose-1,6-bisP and/or is not inhibited by AMP.
According to a ninth aspect of the present invention there is provided a vector for the transformation of an organism, especially a plant or a plant cell, to increase the AGP content of such an organism consequently to increase the rate of starch production by such an organism, such vector comprising a recombinant DNA construct containing a DNA sequence encoding an exogenous ADP glucose pyrophosphorylase enzyme (AGP), such vector also incorporating the necessary promoter and other sequences enabling the expression of that exogenous AGP in an organism transformed by that vector, characterised in that the said DNA sequence is the gene sequence encoding the barley (Hordeum vulgare) endosperm AGP or a sub-unit thereof, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more points in the amino acid sequences defining the barley endosperm AGP or either of its sub-units, wherein the construct is capable of being expressed in the organism; characterised in that the activity of the enzyme or sub-unit thereof is substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of inorganic phosphate; and further characterised in that the activity of the enzyme or sub-unit thereof is not stimulated by fructose-1,6-bisP and/or is not inhibited by AMP.
According to a tenth aspect of the present invention there is provided a method of targeting an exogenous protein to the amyloplast of plants or plant cells which comprises introducing into the plant or plant cell a recombinant DNA construct containing a DNA sequence encoding a starch branching enzyme transit peptide and an exogenous DNA sequence encoding the exogenous protein; wherein the construct is capable of being expressed in the plant or plant cells; preferably wherein the DNA sequence encoding the starch branching enzyme comprises the sequence identified as SEQ ID No.5 and/or the starch branching enzyme expressed in the plant or plant cell by said construct comprises the amino acid sequence identified as SEQ ID No.6.
According to an eleventh aspect of the present invention there is provided an AGP enzyme or sub-unit thereof whose in vivo activity is substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of Pi, and whose activity is not stimulated by fructose-1,6-bisP and/or is not inhibited by AMP.
According to a twelfth aspect of the present invention there is provided a foodstuff made from or comprising an organism according to the present invention; preferably wherein the foodstuff is a fried foodstuff; more preferably wherein the foodstuff is a potato.
The term xe2x80x98transgenic organismxe2x80x99 in relation to the present invention means an organism comprising an expressable exogeneous nucleotide sequence or an expressed product of such an expressable exogeneous nucleotide sequence. Preferably the expressable exogeneous nucleotide sequence is incorporated in the genome of the organism.
The term xe2x80x98organismxe2x80x99 in relation to the present invention includes any starch producing organisms such as plants, algae, fungi and bacteria, as well as cells thereof. Preferably the term means a plant or cell thereof, more preferably a potato and especially a potato tuber.
The term xe2x80x98nucleotidexe2x80x99 in relation to the present invention includes genomic DNA, cDNA, synthetic DNA, and RNA. Preferably it means DNA, more preferably cDNA.
The terms xe2x80x98allelexe2x80x99 and xe2x80x98variantxe2x80x99 in relation to the present invention mean any substitution of, variation of, modification of, replacement of, deletion of or the addition of one or more nucleic acid(s)/amino acids from or to the sequence providing the resultant sequence expresses or exhibits the required enzymatic activity. They also mean a substantial homologous sequence wherein there is homology with respect to at least the essential nucleic acids/nucleic acid residues/amino acids for expression of or exhibition of the required enzymatic activity. Preferably there is at least 80% homology, more preferably at least 90% homology, and even more preferably there is at least 95% homology with the listed sequences. Hybrid sequences are also covered. These may be prepared from at least two different sourcesxe2x80x94e.g. the variant may include a sequence from one source that gives the variant the independence vis-a-vis the level of in vivo 3-phospho-glycerate and a sequence from another source that gives the variant the independence vis-a-vis the level of in vivo Pi.
The term xe2x80x98sub-unitxe2x80x99 in relation to the present invention means an active component of the enzyme that displays the required enzymatic activity. For example, in the case of AGP enzyme obtained from barley (Hordeum vulgare), which is a heterotetramer containing two large sub-units and two small sub-units, the term includes any one of those sub-units as well as combinations thereof as well as a shortened variant thereof.
The term xe2x80x98retains enzymatic activityxe2x80x99 in relation to the present invention means that the activity of the enzyme variant or sub-unit thereof is still substantially independent of any level of in vivo 3-phospho-glycerate and/or any in vivo level of inorganic phosphate, but not necessarily to the same extent as the native enzyme.
The term xe2x80x98substantially independentxe2x80x99 in relation to the present invention means that the enzyme has a decreased sensitivity to levels of PGA and/or of inorganic phosphate, preferably at least to PGA. By way of example, in the absence of PGA the levels of AGPase activity of the native enzyme or sub-unit thereof are in the order of at least 0.002 units per mg protein, preferably at least 0.01 units per mg proteinxe2x80x94when measured in Bintje potato tuber extract. Typically, in the case of the preferred barley AGP enzyme we have found that the AGPase levels of the large sub-unit are greater than the levels of the small sub-unit and are typically in the order of greater than 0.02 units per mg protein and can be in the order of 0.05 units per mg proteinxe2x80x94when measured in Bintje potato tuber extract. This is in contrast to the known enzymes which have no, or at most negligible, AGPase activity in the absence of PGA.
Preferably the enzymatic activity of the AGP enzyme is at least substantially independent of any level of in vivo 3-phospho-glycerate.
More preferably the enzymatic activity of the AGP enzyme is not stimulated by fructose-1,6-bisP and it is not inhibited by AMP.
Preferably the AGP enzyme is a heteromer, preferably a heterotetramer, more preferably a heteromer containing two large sub-units and two small sub-units.
Preferably the AGP enzyme is isolatable from Hordeum, preferably wherein the enzyme is barley (Hordeum vulgar) endosperm AGP or a sub-unit thereof, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more points in the amino acid sequences defining the barley endosperm AGP or either of its sub-units.
Preferably the nucleotide sequence is a DNA sequence.
Preferably the DNA sequence encodes the large sub-unit of the barley endosperm AGP or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more points in the amino acid sequence defining the large sub-unit of the barley endosperm AGP.
Preferably the DNA sequence is the sequence identified herein as SEQ ID No. 1, including non-critical allelic variations of that sequence.
Preferably the DNA sequence encodes the small sub-unit of the barley endosperm AGP, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more points in the amino acid sequence defining the small sub-unit of the barley endosperm AGP.
Preferably the DNA sequence is the sequence identified herein as SEQ ID No. 2, including non-critical allelic variations of that sequence.
Preferably both of the DNA sequences are expressed in the same organism. The DNA sequences need not be derived from the same initial source, such as barley. It is preferred however that they are from the same source, for example barley.
Preferably, when both of the DNA sequences are expressed in the same organism, each DNA sequence additionally codes for a different markerxe2x80x94e.g. the large or small sub-unit of barley AGP enzyme may be in a construct that contains a kanamycin resistance gene such as a construct based on plasmid pBKL4 or pVictor IV GN and another small or large sub-unit of barley AGP enzyme may be in a construct that contains a mannose isomerase gene such as a construct based on plasmid pVictor IV SGiN Man.
Preferably the expressed AGP enzyme or sub-unit thereof comprises the amino acid sequence set out in SEQ ID No. 3, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the expressed AGP enzyme or sub-unit thereof comprises the amino acid sequence set out in SEQ ID No. 4, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the expressed ACP comprises both a large sub-unit having the amino acid sequence set out in SEQ ID No. 3 or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence, and a small sub-unit having the amino acid sequence set out in SEQ ID No. 4 or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the nucleotide sequence additionally codes for a transit peptide which can transport, or assist in the transportation of, the enzyme or sub-unit thereof from the cytoplasm to the relevant or appropriate plastid(s), such as a chloroplast and/or an amyloplast. Preferably the transit peptide is Rubisco Activase transit peptide or Starch Branching enzyme transit peptide.
Preferably the transit peptide is coded for by a DNA sequence comprising the sequence identified as SEQ ID No. 5, including non-critical allelic variations of that sequence.
Preferably the transit peptide has an amino acid sequence comprising the sequence identified as SEQ ID No 6, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the nucleotide sequence is operatively connected to a promoter which expresses the sequence wherein the promoter is cell, tissue or organ specific.
Preferably the promoter has the sequence identified as SEQ ID No. 7, or a variant thereof having non-critical nucleotide substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the AGP enzyme or sub-unit thereof comprises the amino acid sequence set out in SEQ ID No. 3, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence and/or the amino acid sequence set out in SEQ ID No. 4, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in that sequence.
Preferably the organism is a transgenic plant.
Preferably the transgenic plant is a potato plant.
Preferably the nucleotide sequence according to the present invention is obtainable from any one of the following plasmids: Plasmid pVictor IV SGiN Man; Plasmid pPPS 1; Plasmid pPPL1; Plasmid pPPL1M; Plasmid pPPS4; Plasmid pPPL4; Plasmid pPPL5; Plasmid pBKL4; Plasmid pVictor IV GN.
Preferably the enzyme is obtainable from a eukaryotic source.
The present invention has broad applicability to starch producing organisms, especially plants. The present invention works better in organisms such as plants compared to bacteria.
In particular the present invention works better in plants compared to E. coli where AGP activity is stimulated by fructose-1,6-bisP and inhibited by AMP. This E. coli pathway is different to the pathway for the biosynthesis of starch in plants and algae.
With regard to one preferred aspect of the present invention, namely a foodstuff prepared from frying a potato according to the present invention, it is to be noted that the increased starch content of the potato will lead to less fat/oil uptake during frying This results in obvious dietary advantages. Moreover, the increased levels of starch also means that there are decreased free levels of reducing carbohydratesxe2x80x94which are used in starch synthesisxe2x80x94and so there is a decreased tendancy for the resultant product to become discoloured on frying by example reaction of the reducing carbohydrates with the hot fat/oil.
In accordance with a preferred aspect of the present invention it was found that AGP from barley (Hordeum vulgare) endosperm is highly active even in the absence of the activator PGA and is relatively insensitive to PGA/P; ratios which play an important regulatory function in the case of AGP from most other known plant sources. The PGA/Pi; ratio is also believed to play an important regulatory function in non-plant AGP, e.g. algal AGP.
The cDNA sequences encoding parts of the large and the small sub-units of the barley endosperm AGP and the deduced amino acid sequences have recently been established and published in Plant Molecular Biology, 19, 381-389 (1992). The complete DNA sequence encoding the large sub-unit together with the cDNA for the large sub-unit are set out in Plant Physiol. 100 1617-1618, (1992).
In accordance with the present invention the complete DNA sequence encoding the small sub-unit of the barley endosperm AGP and the deduced amino acid sequence has now been established. Those complete cDNA sequences are reproduced herein as SEQ ID Nos. 1 and 2 encoding, respectively, the large and small sub-units of the barley endosperm AGP, whilst the deduced amino acid sequences are set out herein as SEQ ID Nos. 3 and 4, respectively.
Thus, in the preferred embodiment of the present invention, it was discovered that starch production in plants can be enhanced/increased by incorporating into the plant""s genome and under the control of suitable promoter sequence or sequences promoting the expression of the gene in the plant cells, particularly in the chloroplasts and amyloplasts, DNA sequences encoding either the large (60 kDa) sub-unit of barley (Hordeum vulgare) endosperm AGP or the small (51 kDa) unit, or both.
Thus, in a highly preferred aspect of the present invention there is provided transgenic plants and plant cells having increased rates of starch production and/or starch content, as compared with the corresponding non-transformed plant or plant cell, such plants and plant cells having been transformed with a recombinant DNA construct containing, in operational relationship (particularly in downstream relationship) to a plant promoter sequence or sequences enabling the expression of the gene in the plant or plant cell, the gene sequence encoding the barley (Hordeum vulgare) endosperm AGP or an active sub-unit thereof retaining the enzymatic activity of the heterotetrameric AGP, or a variant thereof having non-critical amino acid substitution(s) or deletion(s) at one or more locations in the amino acid sequences defining the barley endosperm AGP or either of its sub-units.
Whilst, in accordance with the present invention, a wide variety of organisms (e.g. plants and plant cells) may be transformed (especially with the gene encoding barley endosperm AGP or either of its sub-units) to increase starch production and starch yields in that particular organism, the preferred embodiment concerns the transformation of the major starch producing plant crops, namely potato, rice, wheat and maize, which four crops in terms of calorific value, probably account for three quarters of the world""s food supply. Sugar beet may also be transformed.
In a more specific aspect of the present invention there are provided transgenic plants and plant cells having increased rates of starch production and/or providing increased starch yields compared with the non-transformed material, such plants and plant cells having been transformed with a recombinant DNA construct containing in downstream relationship to a plant promoter sequence or sequences enabling the expression of the gene in the transformed plant or plant cells, either or both the sequences SEQ ID No. 1 and SEQ ID No. 2 as set out in the prescribed fashion in the sequence listings annexed hereto and which are taken to be part of the present specification, or an allelic variant of either sequence showing substantial homology with the listed sequence and containing non-critical nucleotide substitutions at one or more locations in the nucleotide chain.
Alternatively defined, there are provided, in accordance with the present invention, transgenic plants and plant cells showing enhanced levels of AGP production, particularly, in the chloroplasts and amyloplasts, such plants and plant cells having been transformed with a recombinant DNA construct enabling the expression within the plant or plant cells of barley endosperm AGP or either of its sub-units, those subunits having the derived amino acid sequences set out in SEQ ID Nos. 3 and 4, or a variant of such a sequence having non-critical amino acid substitution(s) or deletion(s) at one or more locations in the amino acid sequence defining the barley endosperm AGP or either of its sub-units.
Also provided in accordance with this invention are plant transformation vectors for the transformation of plants and plant cells to increase the AGP content of such plants and plant cells and thus to increase the rate of production of starch by the transformed plant or plant cell and/or the starch yield, such vectors containing one or more promoter sequences functional in plants linked in operational relationship with a DNA sequence encoding barley endosperm AGP, or either of its sub-units. More especially plant transformation vectors are provided comprising one or more promoter sequences functional in plants linked in operational relationship with either or both the sequences SEQ ID No. 1 or SEQ ID No. 2, or an allelic variant of either sequence showing substantial (at least 80%) homology with the listed sequence but having non-critical nucleotide substitution(s) at one or more locations in the nucleotide chain.
With regard to the promoter, numerous promoters which are functional in plants are known. The promoter should be capable of allowing sufficient expression to result in the desired increase in starch production. Preferably, the promoter should be chosen so that the increased starch production is carried out in the plant tissues where the starch production is required. For instance the promoters of starch biosynthetic genes from plants may be useful.
Known examples of such promoters include the promoter of the granule bound starch synthase gene from potato (Van der Leij et al. [1991] Mol. Gen. Genet. 228: 240-248), and the promoter of the starch branching enzyme gene Sbe 1 from rice (Kawasaki et al. [1993] Mol. Gen. Genet. 237: 1-16).
For expression in potato, a tuber specific class I patatin promoter is preferred (Mignery et al. [1988] Gene. 62: 27-44). The DNA sequence encoding the tuber specific class I patatin promoter is set out in the appendix hereto as SEQ ID No. 7. This patatin promoter was obtained from Dr. William Belknap, USDAxe2x80x94ARS, Alabany, Calif.
The DNA sequence encoding barley endosperm AGP is preferably linked to other control sequences for the expression of the DNA in addition to a promoter sequence such as a transcription terminator sequence. Transcription terminators may be derived from a variety of different genes, including plant, viral and Agrobacterium genes. A cauliflower mosiac virus 35S terminator is preferred.
AGP activity can occur in different sites in plants. For example in potatoes AGP activity is mainly localised in the chloroplasts (i.e. plastids specialising in photosynthesis) or the amyloplasts (i.e. plastids specialising in starch storage). Many amyloplast-localised proteins are expressed as precursors and are targeted to the amyloplast by an appropriate transit peptide that is subsequnetly removed. Similarly, many chloroplast-localised proteins are expressed as precursors which can be targeted to the chloroplast by an appropriate target peptide.
Whilst not wishing to be bound by any theory, it is believed that both the large and small sub-units of the barley endosperm AGP are synthesised as precursor peptides. Additional sequences are found to be attached to the amino-termini of the mature proteins which are understood to represent transit peptides. The transit peptide is then cleaved upon sequestration of the presursor protein into the plastid. It is understood that the enzyme is not subjected to any other post-translation modification process in vivo.
However, in another embodiment of the present invention, it is desirable for the AGP transit peptides to be supplemented with one or more additional transit peptides. The transit peptide can be conveniently fused directly at the amino terminal methionine of the AGP barley sub-unit. In further preferred embodiments the barley AGP transit peptide can be substituted by another amyloplast or chloroplast transit peptide. The barley endosperm AGP cDNA is inserted into a convenient cloning vector, e.g. a plasmid, at a suitable restriction site. The DNA sequence of interest can be encloned into further vectors, if necessary, for the incorporation of additional DNA sequences. Suitable plant transit peptides include known chloroplast (Gavel and Von Heine [1990] FEBS Lett. 261: 455-458) or amyloplast (Van der Leij et al. [1991] Mol. Genet. 228: 240-248; Klosgen et al. [1989] Mol. Gen. Genet. 217: 155-161; Brisson et al. The Plant Cell [1989] 1: 559-566) transit peptides.
In potatoes, preferably a rubisco activase transit peptide (Werneke et al. Proc. Natl. Sci. USA [1988] 85: 787-791) or a starch branching enzyme transit peptide is used. The 480 bp starch branching enzyme cDNA sequence from potato showing 120 nucleotides of the 51 untranslated region and 360 nucleotides of the coding region (see SEQ ID No. 5), which contains a putative 75 amino acid transit peptide and 45 amino acids of the mature branching enzyme is set out in the appendix hereto as SEQ ID No. 6.
In addition to the transit peptide portion of a protein, it may be desirable to include sequences encoding a portion of the mature plastid-targeted protein to further facilitate intracellular transport.
Preferably the plasmids are also provided with selection markers to enable the transformed plant cells to be separated out from plant cells which have not been transformed. Suitable genes are known and include e.g. a neomycin phosphotransferase gene (e.g. neo npt II), a phosphinotricine/bialaphos acetyl-tranferase gene (e.g. bar) and a xcex2-glucuronidase gene (e.g. uidA) or a phosphomannose isomerase gene (e.g. manA, pmi).
In a preferred embodiment, the transformation vectors may be prepared by initially obtaining cDNA encoding the small and large units of barley endosperm AGP by the method described in Plant Molecular Biology, 19, 381-389 (1992). For ligation into a convenient cloning vector, e.g. a plasmid, the barley endosperm AGP cDNA is provided with restriction sites at each end by PCR using the oligonucleotide primers obtained by conventional oligonucleotide synthesis procedures or a commercially available oligonucleotide synthesizer such as, for example, Applied Biosystems 381 A DNA synthesizer. These restriction sites should be homologous with sequences in the cloning vector. The desired DNA sequence can be recloned into further vectors for preparation of the ultimate transformation vectors for preparing the transgenic starch producing organism, especially a transgenic plant.
In the preferred embodiment of the present invention, the plant or plant cells may be transformed by any suitable technique for transforming cellsxe2x80x94such as use of T-DNA, electroporation, injection, DNA bombardment or fusion. After transformation, a whole plant can be cultivated from a transformed plant cell in the usual manner.
Preferably, transformation of the plant cell is achieved with T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes. If agrobacteria are used for transformation, the barley endosperm cDNA needs to be incorporated initially into either an intermediate vector or a binary vector. The intermediate vectors can be integrated into Agrobacterium tunefaciens by means of a helper plasmid. Preferably binary vectors are used, which can be transformed directly into agrobacteria. Binary vectors comprise a selection marker gene and a linker or polylinker which are framed by the right and left T-DNA border regions. The agrobacteria used as host cell should comprise a plasmid carrying a vir region, which is necessary for the transfer of the T-DNA into the plant cell. Transformation using Agrobacterium is achieved by cultivating the Agrobacterium with the plant cell.
Depending on the plant species to be transformed, a variety of different plant transformation vectors can be used. These include pBIN 121, pAL4404, pEHA101, pBKL4, pVictor IV SGiNMan and pVictor IV GN.
For the transformation of potato species Agrobacteria the preferred plant transformation vectors are the plasmids pBKL4, pVictor IV SGiNMan and pVictor IV GN. These plasmids are described later in greater detail.
Preferred plasmids used in the construction of the plasmid used for transformations include pPATA1 and pBluescript II KS. Plasmid pBluescript II KS is a widely used cloning vector available from Stratagene.
Plants can be confirmed as transformed by performing conventional blotting assays and PCR.
The starch content of the plants can be analysed based upon the specific gravity determined using the weight in water and the weight in air as described by W. A. Gould In: Chipping Potato Handbook, ed. Gould, W. A. The Snack Food Association, Vermont, 1989, pp 18-22, in an article entitled xe2x80x9cSpecific gravity, its measurement and usexe2x80x9d.
The limitation of the exogenous ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof which retains the enzymatic activity of the AGP enzyme being not stimulated by fructose-1,6-bisP and/or not inhibited by AMP, which further distinguishes the present invention from the AGP enzymes of the prior art such as those of WO 91/19806 and WO 92/11382, can be expressed in the alternative as either the exogenous ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof which retains the enzymatic activity of the AGP enzyme not being only just an E. coli AGP enzyme, or the exogenous ADP glucose pyrophosphorylase (AGP) enzyme or a sub-unit thereof which retains the enzymatic activity of the AGP enzyme being capable of catalysing the reaction
xcex1-glucose-1-P+ATP⇄ADP-glucose+PP1
The following samples were deposited in accordance with the Budapest Treaty at the recognised depositary The National Collections of Industrial and Marine Bacteria Limited (NCIMB) at 23 St. Machar Drive, Aberdeen, Scotland, United Kingdom, AB2 1RY on Mar. 29, 1994:
E. coli containing plasmid pPPS1 (NCIMB 40618);
E. coli containing plasmid pPPL1 (NCIMB 40619);
E. coli containing plasmid pPPS4 (NCIMB 40620);
E. coli containing plasmid pPPL4 (NCIMB 40621); and
E. coli containing plasmid pPPLS (NCIMB 40622).
The following samples were deposited in accordance with the Budapest Treaty at the recognised depositary The National Collections of Industrial and Marine Bacteria Limited (NCIMB) at 23 St. Machar Drive, Aberdeen, Scotland, United Kingdom, AB2 1RY on Mar. 31, 1994:
E. coli containing plasmid pBKL4 (NCIMB 40623);
E. coli containing plasmid pVictor IV GN (NCIMB 40624); and
E. coli containing plasmid pVictor IV SGiN Man (NCIMB 40625).
A detailed construction of plant transformation vectors according to the present invention and the transformation of plants and plant cells using those vectors to produce transgenic plants according to this invention having increased rates of starch biosynthesis and/or starch yield will now be described in more detail.
In this regard, the present invention will now be described only by way of example.